1. Field of the Invention
This invention relates to audio encoding transmission systems which transmit audio signals in an encoded form onto communication lines.
2. Prior Art
There are provided a variety of systems that are designed to transmit audio information onto communication lines in an encoded form. According to the conventional technology of the system, reproduction of the audio information cannot be carried out until transmission completes with respect to all pieces of audio information. For this reason, in order to perform reproduction of the audio information which are transmitted in a real-time manner, it is important to select an optimum bit rate of transmission in consideration of frequency bands of communications. In other words, it is important to select an optimum compression rate of coding. When it is demanded to provide a certain degree of sound quality in reproduction of the audio information, for example, it is not possible to raise the compression rate of coding so much. So, the system requires a wait time to wait for completion of the transmission of all pieces of the audio information. To cope with a demand that a user (i.e., a human operator) gives a brief audition to the reproduced audio information for a short period of time, the compression rate of coding is raised to lower the sound quality in reproduction of the audio information.
To meet the above demand, the conventional technology proposes an audio encoding system which is a scalable system that is capable of selecting a certain reproduction quality for decoding as well as a certain compression rate of coding. For example, the technology of the above system is disclosed by the paper entitled "Encoding system, for musical sound and voice, having scalable hierarchical structure", provided by the Japanese people of the names of Mr. Akio Kami and Mr. Satoshi Miki, which is written on pp277-278 of the lecture paper collection, denoted by numerals of "H7.9 Auido B3-1-5", of the Japanese Acoustics Society. This system uses an encoding method (or coding method) using three levels of hierarchy. Herein, a first level of hierarchy encodes narrow-band signals which are obtained by performing down-sampling of input signals. In higher levels of hierarchy, a difference is detected between an input signal, whose band is broadened as compared with a lower level of hierarchy, and reproduced sound; then, encoding is performed on the difference. By the way, a low bit rate is employed when frequency bands of communications (referred to as communication bands) do not have a sufficient room for communications. In contrast, a high bit rate is employed when the communication bands have a sufficient room for communications. The above encoding system employing the hierarchical structure is characterized by changing over pieces of information belonging to the levels of hierarchy. That is, in case of the low bit rate, the system uses only the information of a low level of hierarch to enable encoding at a low quality. In case of the high bit rate, the system uses information of a high level of hierarchy as well, so it is possible to reproduce sound of a broad band with a high quality.
According to the conventional audio encoding transmission system described above, it may be possible to give a brief audition to the audio information at a receiving station (corresponding to a decoding system) for a short period of time. However, to enable transmission of the audio information with a higher quality, it is necessary to start re-transmission of bit streams from their first portions at a high bit rate. Therefore, the conventional system suffers from a problem that a wait time in reproduction should become longer because of the re-transmission of the bit streams at a high bit rate.